Security is a question lurking in our minds when it is any sort of technology but with any technology that is run over the internet the question of security is even more important.
VOIP runs through the internet and therefore the information can be tapped anytime. Although with the progress of technology the VOIP security is tightening much and it would be made extremely safe in the near future.
The important sources of securing the information sent by customers are through the encryption and the tunneling process. These keep the Hackers off and most of the VOIP providers would use the encryption method known as Secure Sockets and also the Layer 2 Tunneling and SSL to keep anyone from tapping the information.
Although these are major areas of concern yet do not stop from getting VOIP services. It is the same question of security when you are sending emails.